Hybrid Composite of Sn(IV)-Porphyrin and Mesoporous Structure for Enhanced Visible Light Photocatalytic Degradation of Organic Dyes

Two hybrid composites (SnP@MCM−41 and SnP@SiO2) were fabricated by chemical adsorption of (trans-dihydroxo)(5,10,15,20-tetraphenylporphyrinato)tin(IV) (SnP) on mesoporous structured Mobil Composition of Matter No. 41 (MCM−41) and SiO2 nanoparticles. These materials were characterized by Fourier-transform infrared spectroscopy, ultraviolet–visible spectroscopy, fluorescence spectroscopy, transmission electron microscopy, and field-emission scanning electron microscopy techniques. The incorporation of SnP into MCM−41 and SiO2 supports efficient photocatalytic degradation of the anionic erioglaucine, cationic rhodamine B, and neutral m-cresol purple dyes under visible light irradiation in an aqueous solution. The performances of degradation of these dyes by these photocatalysts under visible light irradiation varied from 87 to 95%. The pseudo-first-order degradation rate constant of organic dyes for SnP@MCM−41 was higher than those of SnP@SiO2 and SnP. These visible light photocatalysts showed remarkable stability and reliable reusability.


Introduction
The global environment suffers from water pollution due to increased industrial and agricultural activities, which causes serious problems for aquatic life [1,2]. Every year, large amounts of hazardous substances such as dyes, pesticides, pigments, herbicides, phenols, biphenyls, and nitro and amino compounds are discharged into water bodies, impairing the drinking quality of water. In this regard, scientific efforts to remove these toxic substances from wastewater are increasing [3,4]. Numerous physicochemical methods including adsorption [5], filtration [6], precipitation [7], and advanced oxidation processes (AOPs) [8,9] have been employed to remove these pollutants. AOP is the most suitable technology because it is simple, has a low cost and high degradation rate, and degrades the entire pollutant to nontoxic H 2 O and CO 2 without generation of other secondary pollutants. Appropriate photocatalysts in the AOPs absorb visible light from the sun and generate reactive oxygen species in situ, which promote the degradation of chemicals in water. A large number of photocatalysts have been used to analyze the degradation of organic pollutants in water. Porphyrinoids (free-base porphyrins and metalloporphyrins) largely absorb visible light and can be used as visible-light-activated photocatalysts [10,11]. However, the use of porphyrinoids in homogeneous catalysis is limited by their aggregation in solution, small surface area, poor reusability, and difficulty in postseparation of the catalyst from the reaction mixture [12]. In addition, it is necessary to develop new photocatalysts to overcome main challenges such as the reduced bandgap, increased solar absorption, fast electron-hole recombination, and smooth recovery from the reaction medium. To this end, porphyrin-based hybrid composite materials have been promising photocatalysts for the degradation of organic pollutants in water and have attracted considerable attention in the applied chemistry community over the past decades [13][14][15][16][17]. In this method, porphyrinoids are combined with one or more components, leading to the formation of nonaggregated nanomaterials with controlled surface morphologies. These materials exhibit remarkable features compared to starting compounds, including a large surface area, high physiochemical stability, and morphology-dependent physical properties.
In this context, we have been interested in hybrid nanocomposite materials comprising metalloporphyrins and mesoporous supports. In this study, a mesoporous Mobil Composition of Matter No. , which consists of a hexagonally well-ordered structure with large pore size and surface area, was used. The numerous silanol groups on the cavity surface attract other functional groups for chemical or physical adsorption [18][19][20]. Commercial SiO 2 nanoparticles were used for comparative studies. We selected Sn(IV)porphyrins as a source of porphynoid compounds. Sn(IV)-porphyrin was selected over others owing to its unique optical properties and peculiar octahedral geometry. Owing to the oxophilic nature of the Sn(IV) center, Sn(IV)-porphyrin can readily form stable six-coordinated complexes with two oxyanions of alkoxides or carboxylates at the trans position. This advantageous tuning capability of Sn(IV)-porphyrins enables creation of coordination complexes [21][22][23][24][25], multiporphyrin arrays [26][27][28], and nanostructures [29][30][31], which exhibit characteristic functions. In particular, supramolecular nanostructures based on Sn(IV)-porphyrins have been intensively studied for the application of photocatalysis of AOPs for water remediation [32][33][34][35][36][37][38][39][40]. Therefore, the combination of Sn(IV)-porphyrins with the mesoporous MCM−41 not only enables morphology fabrication of the surface but also enhances the photocatalytic degradation efficiencies of organic pollutants by manifold compared to the parent Sn(IV)-porphyrins (Scheme 1). As model organic pollutants, anionic erioglaucine (EG) [41], cationic rhodamine B (RhB) [42], and neutral m-cresol purple (MCP) [43] dyes were chosen to investigate photocatalytic degradation. These pollutants are potentially carcinogenic even at very low concentrations, non-biodegradable, and longlasting in aqueous solutions. These water-soluble dyes have been widely used in the textile, paper-printing, and leather industries and are discharged in significant amounts to the environment. This results in undesirable water pollution, and thus it is required to remove these dyes from wastewater. medium. To this end, porphyrin-based hybrid composite materials have been promising photocatalysts for the degradation of organic pollutants in water and have attracted considerable attention in the applied chemistry community over the past decades [13][14][15][16][17]. In this method, porphyrinoids are combined with one or more components, leading to the formation of nonaggregated nanomaterials with controlled surface morphologies. These materials exhibit remarkable features compared to starting compounds, including a large surface area, high physiochemical stability, and morphology-dependent physical properties.
In this context, we have been interested in hybrid nanocomposite materials comprising metalloporphyrins and mesoporous supports. In this study, a mesoporous Mobil Composition of Matter No. 41 (MCM−41), which consists of a hexagonally well-ordered structure with large pore size and surface area, was used. The numerous silanol groups on the cavity surface attract other functional groups for chemical or physical adsorption [18][19][20]. Commercial SiO2 nanoparticles were used for comparative studies. We selected Sn(IV)-porphyrins as a source of porphynoid compounds. Sn(IV)-porphyrin was selected over others owing to its unique optical properties and peculiar octahedral geometry. Owing to the oxophilic nature of the Sn(IV) center, Sn(IV)-porphyrin can readily form stable six-coordinated complexes with two oxyanions of alkoxides or carboxylates at the trans position. This advantageous tuning capability of Sn(IV)-porphyrins enables creation of coordination complexes [21][22][23][24][25], multiporphyrin arrays [26][27][28], and nanostructures [29][30][31], which exhibit characteristic functions. In particular, supramolecular nanostructures based on Sn(IV)-porphyrins have been intensively studied for the application of photocatalysis of AOPs for water remediation [32][33][34][35][36][37][38][39][40]. Therefore, the combination of Sn(IV)-porphyrins with the mesoporous MCM−41 not only enables morphology fabrication of the surface but also enhances the photocatalytic degradation efficiencies of organic pollutants by manifold compared to the parent Sn(IV)-porphyrins (Scheme 1). As model organic pollutants, anionic erioglaucine (EG) [41], cationic rhodamine B (RhB) [42], and neutral mcresol purple (MCP) [43] dyes were chosen to investigate photocatalytic degradation. These pollutants are potentially carcinogenic even at very low concentrations, non-biodegradable, and long-lasting in aqueous solutions. These water-soluble dyes have been widely used in the textile, paper-printing, and leather industries and are discharged in significant amounts to the environment. This results in undesirable water pollution, and thus it is required to remove these dyes from wastewater.

Fabrication and Characterization
Hybrid composites (SnP@MCM−41 or SnP@SiO 2 ) composed of Sn(IV)-porphyrin and support materials were fabricated through the reaction of SnP with silanol groups of mesoporous materials. Condensation of the axial hydroxo ligand of Sn(IV)-porphyrin with a silanol group of the silica surface forms Sn−O−Si chemical bonds in the fabrication of the hybrid composites, as shown in Scheme 1. In a typical procedure, SnP was reacted with MCM-41 or SiO 2 in dichloromethane at room temperature for 24 h, and then the solid was filtered to obtain a hybrid composite of SnP@MCM-41 or SnP@SiO 2 . The amount of adsorbed Sn(IV)-porphyrin (SnP) was estimated based on the tin content of the hybrid composite, SnP@MCM−41 or SnP@SiO 2 , measured by ICP analysis. The chemically adsorbed amounts of SnP in SnP@MCM−41 and SnP@SiO 2 were estimated to be 0.094 and 0.091 mmol/g, respectively.
The surface modification was characterized by zeta potentials determined by the degree of electrostatic repulsion of the surface. As shown in Table S1 The structural patterns of SnP, SnP@MCM−41, and SnP@SiO 2 were examined by X-ray diffractometer ( Figure S1). It is evident from Figure S1 that the peaks are broad, and the XRD patterns are very similar for a typical amorphous solid. SnP shows two peaks centered at 9.  (Table S2). Therefore, the enhanced light absorptions and narrower bandgaps of SnP@MCM−41 and SnP@SiO 2 can effectively improve solar energy utilization, generating more photo-generated carriers to participate in the photocatalytic degradation reaction.  (Table S2). Therefore, the enhanced light absorptions and narrower bandgaps of SnP@MCM−41 and SnP@SiO2 can effectively improve solar energy utilization, generating more photo-generated carriers to participate in the photocatalytic degradation reaction.     (Table S2). Therefore, the enhanced light absorptions and narrower bandgaps of SnP@MCM−41 and SnP@SiO2 can effectively improve solar energy utilization, generating more photo-generated carriers to participate in the photocatalytic degradation reaction.   As the separation efficiency of photo-generated carriers has an important role in the photocatalytic process, its behavior was investigated by fluorescence spectroscopy. As shown in Figure 3, SnP emits fluorescence at 600 and 652 nm under excitation light with a wavelength of 560 nm, whereas SiO 2 and MCM−41 do not exhibit fluorescence emission. Both hybrid composites similarly exhibited a set of emission bands, at 602 and 652 nm for SnP@MCM−41 and at 603 and 653 nm for SnP@SiO 2 . The emission peak-to-peak ratio of SnP was changed for SnP@MCM−41 and SnP@SiO 2 , which suggests strong adhesion.
SnP@SiO2 experienced apparent weight loss starting at 525 °C, where the surface-bound organic groups of SnP were removed. The TGA observations indicate that SnP@MCM−41 and SnP@SiO2 are thermally stable up to 550 °C.
The structural and morphological development of the fabricated SnP@MCM−41 and SnP@SiO2 was investigated by FE−SEM. The morphologies of SnP@MCM−41 and SnP@SiO2 along with those of the starting materials are presented in Figure 5 and Figure  S2. The FE−SEM images show that SnP@MCM−41 has an average diameter of approximately 100 nm with a narrow size distribution ( Figure 5d). Compared to MCM−41, SnP@MCM−41 has an irregular surface, possibly due to the chemical adsorption of SnP on the surface of MCM−41. On the other hand, the sphere-shaped SnP@SiO2 has an average diameter of approximately 70 nm. Notably, the morphology of SnP does not exhibit a characteristic shape and size. EDS mapping images of SnP@SiO2 and SnP@MCM−41 ( Figures S3 and S4) confirm the homogeneous distribution of elements (C, N, O, Si, and Sn).           Table S3. The BET surface area and average pore size of SnP@MCM−41 are 648.7 m 2 /g and 1.53 nm, respectively, suitable for catalytic applications due to the large available surface. The decreases in the surface area and pore size from the bare MCM−41 to SnP@MCM−41 provide evidence for the incorporation of SnP within the channels of mesoporous frameworks. Therefore, MCM−41 was successfully coupled with SnP to form the SnP@MCM−41 mesoporous hybrid composite that maintained high thermal stability and a large surface area.

Photocatalytic Degradation of Organic Dyes
The photocatalytic performances of SnP@MCM−41 and SnP@SiO2 were investigated by the degradation of the organic dyes under visible light irradiation in aqueous solutions. For this purpose, anionic EG, cationic RhB, and neutral MCP were selected as target pollutant dyes for photocatalytic degradation. After approximately 30 min to reach the adsorption-desorption equilibrium, 5, 18, and 25% of EG was adsorbed by SnP, SnP@SiO2, and SnP@MCM−41, respectively ( Figure S6). This indicates that the large surface area and mesoporous framework of SnP@MCM−41 can promote mass diffusion and enhance adsorption. Time-dependent absorption spectra of EG in the presence of SnP@MCM−41 under visible light irradiation are shown in Figure S7. Negligible decay of the EG dye was observed in the absence of either visible light or photocatalyst SnP@MCM−41 ( Figure S8), which implies that visible light is essential for the degradation of the EG dye in addition to photocatalysts. In Figure S7, the absorbance of the EG dye at 629 nm decreases with the increase in the visible light irradiation time. Figure 7 shows that all prepared catalysts exhibit significant progress in the photodegradation of EG in an aqueous solution.
The degradation of the EG dye in the presence of photocatalysts can be described by its degradation efficiency, (C0 − C)/C0, where C0 is the initial concentration of EG and C is the concentration at time t. The observed degradation rate of EG is 13% for SnP, 87% for SnP@SiO2, and 95% for SnP@MCM−41 within 90 min of irradiation of visible light ( Figure  7). Therefore, the photocatalyst SnP@MCM−41 exhibits better performance toward the degradation of the EG dye than SnP and SnP@SiO2. To further elucidate the reaction kinetics for the decay of the EG dye, we used the pseudo-first-order concept, expressed by ln(C0/C) = kt, which is generally used for a photocatalytic degradation experiment if the initial concentration of the dye is low, where k is the pseudo-first-order degradation rate constant. Based on the data plotted in Figure 7, the reaction kinetics of EG dye degradation are presented in Figure S9. The first-order rate constant for the degradation of the EG dye is 0.002 min −1 for SnP, 0.022 min −1 for SnP@SiO2, and 0.029 min −1 for SnP@MCM−41.

Photocatalytic Degradation of Organic Dyes
The photocatalytic performances of SnP@MCM−41 and SnP@SiO 2 were investigated by the degradation of the organic dyes under visible light irradiation in aqueous solutions. For this purpose, anionic EG, cationic RhB, and neutral MCP were selected as target pollutant dyes for photocatalytic degradation. After approximately 30 min to reach the adsorption-desorption equilibrium, 5, 18, and 25% of EG was adsorbed by SnP, SnP@SiO 2 , and SnP@MCM−41, respectively ( Figure S6). This indicates that the large surface area and mesoporous framework of SnP@MCM−41 can promote mass diffusion and enhance adsorption. Time-dependent absorption spectra of EG in the presence of SnP@MCM−41 under visible light irradiation are shown in Figure S7. Negligible decay of the EG dye was observed in the absence of either visible light or photocatalyst SnP@MCM−41 ( Figure S8), which implies that visible light is essential for the degradation of the EG dye in addition to photocatalysts. In Figure S7, the absorbance of the EG dye at 629 nm decreases with the increase in the visible light irradiation time. Figure 7 shows that all prepared catalysts exhibit significant progress in the photodegradation of EG in an aqueous solution.
The degradation of the EG dye in the presence of photocatalysts can be described by its degradation efficiency, (C 0 − C)/C 0 , where C 0 is the initial concentration of EG and C is the concentration at time t. The observed degradation rate of EG is 13% for SnP, 87% for SnP@SiO 2 , and 95% for SnP@MCM−41 within 90 min of irradiation of visible light (Figure 7). Therefore, the photocatalyst SnP@MCM−41 exhibits better performance toward the degradation of the EG dye than SnP and SnP@SiO 2 . To further elucidate the reaction kinetics for the decay of the EG dye, we used the pseudo-first-order concept, expressed by ln(C 0 /C) = kt, which is generally used for a photocatalytic degradation experiment if the initial concentration of the dye is low, where k is the pseudo-first-order degradation rate constant. Based on the data plotted in Figure 7, the reaction kinetics of EG dye degradation are presented in Figure S9. The first-order rate constant for the degradation of the EG dye is 0.002 min −1 for SnP, 0.022 min −1 for SnP@SiO 2 , and 0.029 min −1 for SnP@MCM−41.
The       Figure S12. The pseudo-first-order rate constant for the degradation of MCP is 0.001 min −1 for SnP, 0.021 min −1 for SnP@SiO 2 , and 0.024 min −1 for SnP@MCM−41 (Figures 9 and S13). the effect of the dye/photocatalyst ratio on the degradation of the EG dye, various concentrations of the EG solution (5,10,15,20,25,30,35, and 40 mg L −1 ) were applied with a constant amount (20 mg) of the photocatalyst SnP@MCM−41. The degradation rate decreases with the increase in the concentration of the EG dye. Most of the EG dye is degraded at concentrations of 5−25 mg L −1 , while approximately 60% of the dye is degraded even at 40 mg L −1 ( Figure S19).  The reusability of the photocatalysts SnP@MCM−41 and SnP@SiO 2 is crucial for practical application, which was evaluated by recycling tests on SnP@MCM−41 toward EG dye degradation ( Figure S14). Even after 10 consecutive cycles, SnP@MCM−41 still maintains a high performance for the degradation of the EG dye with a reduction of only 8%, which indicates that the photocatalyst SnP@MCM−41 has remarkable stability. After the degradation reaction, the structure of SnP@MCM−41 was additionally analyzed to confirm the stability of this photocatalyst. As shown in Figures S15 and S16, the FTIR spectrum and FE−SEM image of the used SnP@MCM−41 closely resembles that of the as-prepared state, which indicates that the mesoporous framework of this photocatalyst was intact during the photocatalytic reaction. Moreover, the process for the recovery of these photocatalysts, SnP@MCM−41 and SnP@SiO 2 , from the reaction mixture was very simple through successive filtration, washing, and drying procedures.
We optimized the reaction conditions in terms of the EG dye/photocatalyst ratio, temperature, and pH of the solution. Photodegradation experiments were carried out at different temperatures to evaluate the effect of temperature on the degradation of the EG dye by the photocatalyst SnP@MCM−41. With the increase in temperature, the degradation efficiency increases up to 50 • C ( Figure S17). The pH of the aqueous EG dye solution apparently affected the degradation rate of the EG dye, as shown in Figure S18. The rate of degradation increases from pH = 2 to pH = 8, and then decreases to pH = 12. To evaluate the effect of the dye/photocatalyst ratio on the degradation of the EG dye, various concentrations of the EG solution (5, 10, 15, 20, 25, 30, 35,  were used for the measurement of the EG dye degradation rate ( Figure S20). The photodegradation rate of EG for composites SnP@MCM−41 (0.01 mmol/g to 0.2 mmol/g) was higher than pure SnP. The rate also increased when the molar amount of SnP compared to MCM−41/g increased in the composite, reaching the maximum of 0.1 mmol/g. After that, the rates decreased from 0.15 mmol/g to 0.2 mmol/g. When the molar amount of SnP to MCM−41/g is lower than 0.05 mmol/g, the host photocatalyst SnP has low content in the composite to reduce the amount of photogenerated carriers, resulting in relatively low catalytic efficiency. When the molar amount ratio is higher than 0.1 mmol/g, SnP significantly agglomerates on the MCM−41 surface, reducing reactive sites, suppressing photogenerated charge separation, and gradually reducing degradation efficiency of EG dye. This suggests the synergistic effect between SnP and MCM−41 is responsible for the noticeably enhanced photocatalytic activity of the composite SnP@MCM−41.
We previously reported a possible mechanism for the photodegradation of organic dyes by porphyrin-based nanomaterials in an aqueous solution under visible light irradiation [38,39]. Usually, the mechanism includes five steps. In step 1, a photocatalyst in an aqueous solution absorbs light upon exposure to visible light irradiation along with the EG dye. Valence-band electrons are promoted to the conduction band after crossing the bandgap. This assists in the generation of electron-hole (e − /h + ) pairs at the surface of the photocatalyst. These photogenerated holes (h + ) react with H 2 O to form a highly reactive hydroxyl radical ( • OH) (step 2). On the other hand, the excited electron reacts with the dissolved O 2 to generate highly reactive superoxide radical anions (O 2 −• ) in step 3. These photogenerated, highly reactive superoxide radical anions and hydroxyl radicals react with the EG dye and degrade it into small molecules and finally to CO 2 and H 2 O (steps 4 and 5). The photocatalytic degradation mechanism of 4-chlorophenol and acid orange 7 by SnP-based photocatalysts was similarly discussed [14].
The mechanism consists of five steps for a porphyrin-based porous framework P: • OH + EG → degraded products (4) Radical trapping experiments were carried out to detect the photogenerated reactive species during the photocatalytic degradation of the EG dye [44,45]. We used ethylenediaminetetraacetic acid disodium (Na 2 EDTA) to capture holes, tert-butanol ( t BuOH) to capture hydroxyl radicals ( • OH), and para-benzoquinone (p-BQ) for superoxide radical anions (O 2 −• ) during the photodegradation experiment of the EG dye in the presence of SnP@MCM−41. Figure S8 confirms that the EG dye degradation rate is critically affected in the presence of t BuOH as well as p-BQ. Superoxide radicals (O 2 −• ) are a major reactive species compared to hydroxyl radicals ( • OH) responsible for the catalytic degradation of the EG dye in an aqueous solution. However, the degradation of the EG dye was not affected by the presence of Na 2 EDTA or photogenerated holes.
We also analyzed the degradation products of the EG dye after visible light irradiation in the presence of the photocatalyst SnP@MCM−41. The reaction mixture was analyzed by electrospray ionization-mass spectrometry (ESI-MS) after 45 min for each photodegradation experiment ( Figure S21). New peaks in the mass spectra confirmed the degradation of the EG dye to new small molecules [46]. Based on the mass spectra in Figure S16, possible intermediates for the degradation of the EG dye are shown in Figure 10. Initially, the base peak (m/z = 769 for [EG−Na]) and another peak (m/z = 373 for [EG−2Na]) belong to the anionic EG dye. The EG dye can be fragmented in two different manners. The first is through cleavage of the tertiary amine bond and formation of two low-molecularweight fragments with a secondary amine (m/z = 577) and 3-methyl-benzenesulfonic acid (m/z = 171). This secondary amine can further undergo hydrolysis and generate a tertiary alcohol derivative (m/z = 594). Again, successive oxidation of tertiary alcohol leads to formation of lower-molecular-weight fragments (m/z = 339 and 93). In the second process, the cleavage of the C-C double bond leads to formation of two lower-molecular-weight amine fragments (m/z = 459 and 290). These low-molecular-weight amines can then undergo successive hydrolysis that leads to formation of benzenesulfonic acid (m/z = 157). Finally, these lower-molecular-weight intermediate molecules were further fragmented and mineralized into CO 2 and H 2 O. The total organic carbon (TOC) value was calculated to estimate the removal of the EG dye by photocatalysts [47]. The TOC removal percentage attained using SnP@MCM−41 was only 75%.
degradation of the EG dye to new small molecules [46]. Based on the mass spectra in Figure S16, possible intermediates for the degradation of the EG dye are shown in Figure 10. Initially, the base peak (m/z = 769 for [EG−Na]) and another peak (m/z = 373 for [EG−2Na]) belong to the anionic EG dye. The EG dye can be fragmented in two different manners. The first is through cleavage of the tertiary amine bond and formation of two low-molecular-weight fragments with a secondary amine (m/z = 577) and 3-methyl-benzenesulfonic acid (m/z = 171). This secondary amine can further undergo hydrolysis and generate a tertiary alcohol derivative (m/z = 594). Again, successive oxidation of tertiary alcohol leads to formation of lower-molecular-weight fragments (m/z = 339 and 93). In the second process, the cleavage of the C-C double bond leads to formation of two lower-molecularweight amine fragments (m/z = 459 and 290). These low-molecular-weight amines can then undergo successive hydrolysis that leads to formation of benzenesulfonic acid (m/z = 157). Finally, these lower-molecular-weight intermediate molecules were further fragmented and mineralized into CO2 and H2O. The total organic carbon (TOC) value was calculated to estimate the removal of the EG dye by photocatalysts [47]. The TOC removal percentage attained using SnP@MCM−41 was only 75%. Finally, the degradation performances of organic dyes represented by the pseudofirst-order rate constants for SnP@MCM−41 and SnP@SiO2 were compared to those of Finally, the degradation performances of organic dyes represented by the pseudofirst-order rate constants for SnP@MCM−41 and SnP@SiO 2 were compared to those of selected photocatalysts reported in the literature. As listed in Table 1, SnP@MCM−41 and SnP@SiO 2 under visible light irradiation exhibit higher than or comparable degradation performances for EG, MCP, and RhB to those of most reported photocatalysts.

Materials and Methods
MCM−41 and SiO 2 nanoparticles were purchased from Sigma-Aldrich. All other chemicals were purchased from TCI and used without further purification unless otherwise specified. (trans-Dihydroxo)(5,10,15,20-tetraphenylporphyrinato)tin(IV) (SnP) was prepared similarly according to the previously reported procedure [59]. CH 2 Cl 2 and pyrrole were distilled from a solution over calcium hydride. Fourier-transform infrared (FTIR) spectra were acquired using a Shimadzu FTIR−8400S spectrophotometer (Shimadzu, Tokyo, Japan). Steady-state ultraviolet-visible (UV-vis) spectra were recorded using a Shimadzu UV-3600 spectrophotometer (Shimadzu, Tokyo, Japan). Fluorescence spectra were recorded using a Shimadzu RF-5301PC fluorescence spectrophotometer (Shimadzu, Tokyo, Japan). Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker AXS D8 Advance powder X-ray diffractometer (Bruker, Billerica, MA, USA). A thermogravimetric analysis (TGA) was performed using an Auto-TGA Q500 instrument (TA Instruments, New Castle, DE, USA). The Brunauer−Emmett−Teller (BET) surface area was determined using an analyzer (BELSORP-mini volumetric adsorption equipment) through N 2 adsorption isotherms at 77 K. The surface area and pore size were estimated using an Autosorb-iQ and Quadrasorb SI. Field-emission scanning electron microscopy (FE−SEM) and transmission electron microscopy (TEM) images were acquired using a MAIA III (TESCAN, Brno, Czech Republic) and JEOL/JEM 2100, respectively. Zeta potentials were measured with an Otsuka Electronics ELSZ−2. Dynamic light scattering (DLS) experiments and inductively coupled plasma (ICP) analysis were performed using a NanoBrook 90Plus DLS size analyzer (Brookhaven, NY, USA) and ICP−Spectrociros CCD instrument, respectively.

Fabrication of SnP@MCM−41
Dried MCM−41 (3.0 g) was added into a solution of SnP (0.23 g) dissolved in CH 2 Cl 2 (100 mL,~3.0 mM) and stirred for 24 h at room temperature. The solids were filtered and washed three times successively with acetone, CH 2 Cl 2 , and N,N−dimethylformamide. Afterward, the solids were dried in a vacuum oven for 4 h at 80 • C, which yielded a powder of SnP@MCM−41 (2.864 g).

Fabrication of SnP@SiO 2
Using dried SiO 2 (3.0 g), a powder of SnP@SiO 2 (3.003 g) was obtained by the same procedure.

Photocatalytic Degradation Experiment
The photocatalytic efficiencies of these hybrid composites were investigated by photodegrading EG, RhB, and MCP dyes in each aqueous solution. In a typical procedure, 20 mg of the photocatalyst was added to 250 mL of an aqueous solution of EG (20 mg L −1 , distilled water with pH of 7.0) with stirring at 298 K. The reaction mixture was allowed to stand in dark for 30 min to reach the adsorption−desorption equilibrium. Afterward, the irradiation process was started using a 150 W xenon arc lamp with a UV cut-off filter (ABET Technologies, Old Gate Lane, Milford, CT, USA) at 298 K. After irradiation with visible light, a 3 mL suspension was collected at regular intervals. The photocatalyst was separated from the solution by centrifugation and collected by filtration with filter paper. The concentration of EG was determined by measuring the absorbance at 629 nm using a UV-vis spectrophotometer. Similarly, an aqueous solution of RhB (40 mg L −1 ) dye was prepared, and its concentration was determined by assessing the absorbance at 553 nm. For the MCP dye, we used an aqueous solution of MCP (65 mg L −1 ) and determined its concentration at 435 nm.

Conclusions
Hybrid composites of SnP@MCM−41 and SnP@SiO 2 were fabricated by the reaction of (trans-dihydroxo)(5,10,15,20-tetraphenylporphyrinato)tin(IV) (SnP) with the mesostructured MCM−41 and nanoparticle SiO 2 , respectively. The formation of hybrid composites was readily achieved through condensation between the hydroxo ligand of SnP and silanol groups of MCM−41 or SiO 2 . These hybrid composites were fully characterized by FTIR spectroscopy, UV-vis spectroscopy, fluorescence spectroscopy, TEM, and FE−SEM techniques. The incorporated SnP onto the mesoporous structure provided efficient photocatalytic degradation for the anionic EG, cationic RhB, and neutral MCP dyes under visible light irradiation in an aqueous solution. The pseudo-first-order degradation rate constant of organic dyes for SnP@MCM−41 was higher than those of SnP@SiO 2 and SnP. These visible light photocatalysts showed remarkable stability and reliable reusability. Our report on visible light photocatalysts provides a valuable contribution for the treatment of dyed wastewater and encourages the development of highly effective hybrid composites composed of porous materials and visible light photocatalysts for applications in environmental remediation.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28041886/s1, Table S1: Zeta potentials and mobilities of SiO 2 , SnP@SiO 2 , MCM−41, SnP@MCM−41, and SnP; Table S2: Band-gap energies (E g ) calculated by the Tauc plot method; Table S3 Figure S14: Typical catalytic cycle (up to 10 cycles) for the photocatalyst SnP@MCM−41 for the degradation of the EG dye; Figure S15: Comparison of FT-IR spectra of SnP@MCM−41 before and after the degradation of the EG dye; Figure S16: Comparison of FE−SEM images of SnP@MCM−41 before and after the degradation of EG dye; Figure S17: Effect of the temperature on the degradation of the EG dye by SnP@MCM−41; Figure S18: Effect of the pH of the solution of the EG dye for photodegradation by SnP@MCM−41; Figure S19: Effect of the concentration of the EG dye for photodegradation by SnP@MCM−41 (20 mg)